Resistance



Oct. 24, 1933. c zw 1,931,974

RESI STANCE Filed Dec. 9, 1929 2 Sheets-Sheet 2 ATTORN EY Patented Oct. 24, 1953 PATENT OFFICE RE SISTANCE Cornelis ZWikker EindhoVen, Netherlands, as-

signor to Radio Corporation of America, a corporation of Delaware Application December 9, 1929, Serial No. 412,637,

and in the Netherlands January 5, 1929 3 Claims.

This invention is based on the observation that with the metals of the zirconium group the temperature coefficient of the electric resistance is 7 negative throughout a certain region.

If the volt-ampere characteristic of these metals is recorded it is found to possess a certain part where any variations in the current strength result in comparatively small variations in the voltage.

According to the invention the above mentioned metals are used for the manufacture of resistance elements. Owing to their peculiar properties these resistance elements are suitable for various uses, among other things for voltage indication and for voltage control.

It has been found that the peculiar properties hereinbefore referred to can be intensified by the addition of suitably chosen quantities of other substances.

The region of negative temperature-coefficient is found to coincide with a region of extremely high specific heat. This circumstance permits the manufacture of resistances the temperature of which is insensible to fluctuations in the current strength. In this respect zirconium occupies a particular place as it possesses this high value of the specific heat just at a temperature which is adapted as working temperature for oxide cathodes and similar cathodes emitting at comparatively low temperature in thermionic devices. Thus the high specific heat decreases to a marked extent the temperature fluctuations when directly heating with alternating current.

The invention will be more clearly understood by reference to the accompanying drawings on which some diagrams and some embodiments of the invention are illustrated.

Figure 1 shows the progress of the specific-re- I sistance of zirconium according to the temperature.

Figure 2 shows the volt-ampere characteristic of a zirconium wire.

Figure 3 shows the progress of the specific heat according to the temperature.

Figure 4 shows an embodiment of the invention used for indicating small variations in potential.

Figure 5 shows an embodiment of the invention V used for maintaining a voltage constant.

Figure 6 shows the progress of the light intensity of a lamp having a filament of zirconium.

Figures 1, 2, and 3 are clear without any further explanation.

, Referring to Figure l, the curve I has reference to pure zirconium, whereas II has reference to (Cl. 250--27.5) v

zirconium which has added to it a small quantity of aluminum. It is readily seen that the addition of the aluminum renders very abrupt the fall in specific resistance in the region of temperature in the neighborhood of 1100 K. In Figure 2 the current intensities are plotted along the axis of abscissas and the voltages along the axis of ordinates. This figure has reference to a zirconium wire containing a quantity of aluminum. It is obvious that by the use of suitably chosen current-limiting or shunt resistances the slope of the part a-b of the characteristic can be controlled.

Referring to Figure 3, the specific heat of zirconium is represented as a function of the abso- 7i? lute temperature. At a temperature slightly exceeding l100 K. the specific heat rises to about 25 times the normal value so that the heat capacity of the wire at this point is much greater than the normal value of the heat capacity which for the. various metals is not widely different. Wires of this material are suitable for cathodes which are supplied with alternating current. For this purpose a wire of zirconium may be coated I with a layer of high emission material such as S0 oxides of the alkaline earth metals. The emitting layer may be mounted directly on the core or an intermediate layer may be used.

Referring to Figure 4, w is a resistance of zir- V conium wire which is mounted in a bulb b and connected in series with an ammeter A. When the voltage across the terminals u and o fluctuates and if the resistance to is correctly dimensioned, the current intensity across A fluctuates V to a much greater extent so that the system shown in Figure 4 constitutes a highly sensitive indicator for fluctuations in potential.

Figure 5 shows in What manner the resistance element according to the invention may be used for branching a constant voltage off a source of current which is subject to potential fluctuations.

u and v are the terminals of a source of current having a voltage which is not perfectly constant and T is a current limiting resistance. The voltage which is obtained from the terminals 22 and y is practically constant as a result of the described peculiar characteristic of the resistance to. The device shown in Figure 5 may be used, for example, for the supply of filaments of thermionic devices from a direct current circuit and more generally in those cases in which a constant voltage is desired and not directly available.

Figure 6 shows the light intensity of a zirconium lamp dependent upon the voltage. The characteristic is recorded for a zirconium wire iic containing aluminum. The ordinates give the light intensity, the abscissas the voltage. From the figure it is apparent that at an increasing voltage the light intensity makes a leap at about 10.4 volts, whereas at a decreasing voltage the light intensity at 10 volts suddenly approaches to practically zero. Naturally, the value of the voltage at which these leaps occur can be controlled by a judicious choice of the length, the thickness, and the composition of the wire.

A zirconium lamp of the kind referred to may be used as a voltage indicator.

What I claim is:

1. An electron emitting cathode comprising an oxide coating of high electron emissivity on a base of an alloy of zirconium and aluminum which is predominantly zirconium and which decreases in resistance with increase in temperature over a temperature range from about 1000 K. to about 1200 K.

2. An electron emitting cathode comprising an oxide coating of high electron emissivity on a base of an alloy of zirconium and aluminum which is predominantly zirconium and exhibits an abrupt drop in resistance at a temperature slightly above 1100K.

3. An electron emitting cathode of high electron emissivity at a working temperature of about 1000" K. and comprising a metal core which has at said working temperature a negative resistance coeflicient and a specific heat about twenty fold the normal value and which consists essentially of zirconium and a coating on said core of an oxide having high electron emissivity at said working temperature.

CORNELIS ZWIKKER. 

